1. Field of the Invention
The present invention relates to a telecentric illumination system and more particularly to a zoom type of telecentric illumination system capable of continuously changing the area illuminated.
2. Description of the Prior Art
Hitherto, telecentric illumination system has been often used in an optical instrument such as projection examining instrument in order to minimize the affection of focusing error on measurement. Also, in order to obtain always bright projected images irrespective of change of magnification of the objective, various systems for the variation of magnification have been proposed one example of which is the zooming system capable of changing the illuminated area with the change of magnification of the objective lens. However, in general, the objection examining instrument or the like has a particular structure which requires a relatively large distance from the illumination lens to the surface of a sample to be examined, that is, to the focal plane of the projection lens. This requirement constitutes a difficult problem in designing the illumination system useful for such optical instruments. In particular, conventional illumination systems have a problem of inability to effectively use marginal rays of the illumination light.
FIGS. 1A-1C illustrate an example of the conventional arrangement of illumination system wherein FIG. 1A shows the state of illumination for high magnification, FIG. 1B for medium magnification and FIG. 1C for low magnification. As seen in FIGS. 1A-1C, marginal rays available for illumination are gradually decreased with the change of magnification toward high magnification at which the area of sample surface (O) to be illuminated is narrowed. Therefore, the efficiency of illumination is reduced accordingly for high magnification. More particularly, in the case of medium magnification (FIG. 1B) and low magnification (FIG. 1C) the composite focal length of the total illumination system is relatively long and the beam diameter of the illumination light running from the light source (S) to the sample surface (O) is large. In addition, the angle which oblique rays indicated by broken lines forms with the optical axis is relatively small. Consequently, the shortage of marginal rays on the sample surface (O) is not so great in these cases.
In contrast, in the case of high magnification (FIG. 1A), the composite focal length is short and the beam diameter becomes small. The angle which the principal ray (P) of the oblique beam forms with the optical axis is large. The oblique rays are entirely off the sample surface (O) and therefore the efficiency of illumination is reduced to a great extent in this case.
To solve the problem of such poor illumination efficiency for high magnification, most of the conventional apparatus have been obliged practically to employ a condensing illumination system for convenience' sake. However, since the solution can not hold the condition of telecentric illumination, such illumination system has never been satisfactory, in particular, for high precision instruments such as optical measuring instruments.